The present invention relates to the art of gas separation. It finds particular application in the separation of substantially pure oxygen gas from atmospheric air and will be described with particular reference thereto. However, it is to be appreciated that the present invention is applicable to the purification of oxygen from other sources and to the separation or purification of other gases.
Heretofore, oxygen has been separated from atmospheric air by selective adsorption. Atmospheric air was cyclically pumped into one of a pair of beds filled with a physical separation material. The physical separation material, such as 5A zeolite, permitted the less strongly adsorbed molecules such as oxygen and argon, to pass therethrough, but trapped or retained the more strongly adsorbed molecules of nitrogen, carbon dioxide, and water vapor. When the trapping or adsorption capacity of the bed was substantially met, the air was pumped to the second bed while the first bed was evacuated or cleansed of the nitrogen and other adsorbed molecules.
Oxygen composes about 21% of atmospheric air whereas argon composes about 1%. When nitrogen, carbon dioxide, and other larger molecules are removed from atmospheric air leaving substantially only oxygen and argon, the percentages of both argon and oxygen in the separated gas increase about five fold. That is, even if the separator works perfectly, passing only oxygen and argon, the resultant product gas will be 4% to 5% argon and 95% to 96% oxygen. The purity of the resultant oxygen gas is theoretically limited by the argon content of atmospheric air to about 95.7% oxygen and 4.3% argon.
By carefully controlling the cycling of gas between the beds and other operating parameters, about 95% pure oxygen can be generated at the output to the beds. If too much air is passed into the system during a cycle, the adsorption capacity of the beds will be exceeded and the purity of the product oxygen becomes substantially less than the maximum theoretical purity of 95.7% or even the typically attainable 95%.
In the applicant's earlier U.S. Pat. No. 4,869,733, a second stage of purification was added. Specifically, the roughly 95% oxygen, 5% argon mixture was fed into a 4A zeolite bed which adsorbed oxygen but which passed argon. When the 4A bed was substantially saturated with oxygen, the bed was discontinued from the gaseous mixture supply. A downstream pump pumped the adsorbed oxygen from the second stage to an oxygen reservoir.
Another two stage oxygen concentration system is illustrated in U.S. Pat. No. 4,190,424 to Armond, et al. Armond utilized a rather complex valving system which alternately caused one of two carbon sieve beds to receive the oxygen-argon mixture while the other carbon sieve bed was connected with a pump which evacuated the adsorbed oxygen. The pair of carbon sieve beds discharged argon mixed with some oxygen as a waste or secondary product gas. The recovered primary product gas included not only the adsorbed oxygen, but also the mixture of oxygen and argon gas that filled the interstitial voids. The loss of oxygen with the waste argon gas and the recovery of argon with the primary product oxygen both resulted in inefficiencies in the Armond system.
In accordance with the present invention, a method and apparatus are provided for separating substantially pure oxygen efficiently from air.